Hardening of steels by nitriding through the use of ammonia is a long-standing commercial production practice. Typically, this method is used to harden only a thin layer at the surface of the steel body or workpiece, i.e., case hardening, to impart, for example, wear resistance. The scientific fundamentals have in fact been known for somewhat more than 50 years and the method presently widely used was introduced into large-scale operations in the late `20`s. In accordance with that method, an ammonia-containing atmosphere is flowed in contact with a steel workpiece at elevated temperature under conditions favoring formation of nitrides of aluminum, titanium, chromium and other stable nitride-forming alloying elements of the steel, typically as a dispersion of particles of the nitride or nitrides in the steel matrix.
Suitability of the nitriding atmosphere is determined periodically by sampling gasses flowing out of the nitriding chamber and measuring their ammonia content; commonly, for example, by means of a gas burette which measures the fraction of the gas which is soluble in water. These chemical-type tests afford indications of the thermodynamic activity of nitrogen, which is porportional to the ratio pNH.sub.3 /pH.sub.2.sup.3/2, where pNH.sub.3 and pH.sub.2 represent the partial pressures of ammonia and hydrogen in the gas. A common objective in controlling the process is to limit the thermodynamic activity of nitrogen so as to form few or no weak and brittle iron nitrides, but within that limitation to maintain the nitrogen activity at a high enough level to enhance the rate of nitrogen diffusion into the iron lattice, and thereby optimize the size of the nitride particles of aluminum and other alloying elements. Since the dissociation of ammonia is catalyzed by the steel being nitrided and by other metal surfaces in the nitriding chamber, and since the catalytic effectiveness of these surfaces changes during the nitriding process, repeated testing of the ammonia concentration in the gas is required during the course of the nitriding process.
This chemical-type process control does not provide a quantitative guide for changing input gas composition or process variables in immediate response to changes in the catalytic activity of surfaces in the nitriding chamber or retort. Nevertheless, this still is the generally used control procedure in steel nitriding operations.